Faculty of Biology, Philipps University & Max Planck Institute for Terrestrial Microbiology
Biochemistry of anaerobes
Laboratory for Microbiology, Biology, PUM
+49-6421 28 22088
We are interested in enzymes with radicals in their catalytic pathways. Well known members of this group use dioxygen, S-adenosylmethionine and coenzyme B as radical generators. We study 2- and 4-hydroxyacyl-CoA dehydratases that only apply one-electron transfers mediated by flavin and iron-sulfur clusters for generation of ketyl radicals (radical anions). These oxygen sensitive enzymes are involved in amino acid fermentations mainly by clostridia and some other anaerobic Bacteria and Archaea. 2-Hydroxyacyl-CoA dehydratases require activation by an ATP-dependent one-electron reduction catalyzed by an iron-sulfur cluster containing ‘archerase’. The thereby proposed conformational change resembles an archer shooting arrows instead of electrons. The electron for the archerase is supplied by ferredoxin that is reduced by NADH in a novel process called ‘bifurcation’ mediated by a complex of butyryl-CoA dehydrogenase and electron-transfer-flavoprotein (ETF). This complex catalyzes the exergonic reduction of crotonyl-CoA to butyryl-CoA by NADH coupled to the endergonic reduction of two ferredoxins by NADH. Thus the electrons of NADH are split or bifurcated to higher and lower redox potentials that enable a new type of energy conservation in anaerobes.
Trans-glutaconic acid, (E)-pentene-1,5-dioic acid, is an unsaturated C5-dicarboxylic acid that can be reduced to the saturated glutaric acid (pentane-1,5-dioic acid), from which its name was derived. In the chemical industry, both acids have the potential to be used as monomers for biodegradable polyesters. Glutaconic acid might be additionally applied for the formation of polyamides by condensation with suitable αω-diaminoalkanes. The ideal material for biological production of glutaconic acid would be glutamic acid obtained by sugar fermentation. The α-elimination of ammonia from glutamate to glutaconate, however, is chemically not feasible. Nevertheless, some anaerobic bacteria, such as Acidaminococcus fermentans Clostridium symbiosum, catalyze this reaction in five steps during the fermentation of glutamate to ammonia, CO2, acetate, butyrate, and molecular hydrogen. This pathway converts glutamate to ammonia and 2-oxoglutarate, followed by an NADH-dependent reduction to (R)-2-hydroxyglutarate. Glutaconate CoA-transferase mediates the activation to (R)-2-hydroxyglutaryl-CoA, which is converted to (E)-glutaconyl-CoA by the 2-hydroxyacyl-CoA dehydratase (see above). To convert Escherichia coli into a glutaconate producer, we expressed six genes in this organism encoding 2-hydroxyglutarate dehydrogenase, glutaconate CoA-transferase, and the activator of the dehydratase fromA. fermentans, as well as 2-hydroxyglutaryl-CoA dehydratase from C. symbiosum. The new pathway diverts from central metabolism at 2-oxoglutarate. Our data demonstrate that the biological production of 3 mM glutaconate is possible. To obtain higher concentrations, many parameters need to be optimized, e. g., the glucose concentration, expression of the genes coding for the transferase and dehydratase, attenuation of ethanol production, and facilitation of glutaconate export. A further goal is the reduction of glutaconyl-CoA to glutaryl-CoA, possibly by expressing the gene coding for the non-decarboxylating glutaryl-CoA dehydrogenase together with the electron bifurcating ETF from Syntrophus aciditrophicus. This could lead to a glutarate producing strain. Recent experiments on the substrate specificity of the enzymes involved in glutaconate production indicate that this pathway could also be used to reduce 2-oxoadipate to adipate, a constituent of Nylon-6,6 (Figure 1).
1. Buckel W, Zhang J, Friedrich P, Parthasarathy A, Li H, Djurdjevic I, Dobbek H & Martins BM (2012) Enzyme catalyzed radical dehydrations of hydroxy acids. Biochim Biophys Acta Online 8 December 2011
2. Herrmann G, Jayamani E, Mai G & Buckel W (2008) Energy conservation via electron-transferring flavoprotein in anaerobic bacteria. J Bacteriol 190, 784-791.
3. Djurdjevic I, Zelder O & Buckel W (2010) Method for the production of glutaconate. International patent application WO/2010/081885
4. Djurdjevic I, Zelder O & Buckel W (2011) Production of glutaconic acid in a recombinant Escherichia coli strain. Appl Environ Microb 77, 320-322.
5. Parthasarathy A, Pierik AJ, Kahnt J, Zelder O & Buckel W (2011) Substrate Specificity of 2-Hydroxyglutaryl-CoA Dehydratase from Clostridium symbiosum: Toward a Bio-Based Production of Adipic Acid. Biochemistry-Us 50, 3540-3550.